A typical integrated circuit (IC) includes an IC package and a semiconductor device, which is physically and electrically connected within the IC package. The semiconductor device typically includes electrostatic discharge (ESD) protection devices that protect the semiconductor device against ESD events that would otherwise cause damage. Generally, the ESD protection devices are located within the semiconductor device in close proximity to semiconductor device pads, which electrically connect to pins of the IC package.
One conventional ESD protection device provides an ESD clamp (or shunt) between the semiconductor pad to be protected and a reference conductor (i.e., a ground conductor). If power is disconnected (e.g., when the semiconductor device is being handled prior to its installation within an IC package, or when an assembled IC is being handled prior to its installation on a circuit board), the ESD protection device shunts or clamps any positive charge on the pad that is above a particular threshold to the reference conductor. If the power is on (e.g., when the assembled IC is installed on a circuit board and is operational), the ESD protection device is deactivated and an incoming signal on the pad is permitted to pass through to other semiconductor device circuitry, i.e., internal circuits of device. An example of such an ESD protection device is described in U.S. application Ser. No. 08/979,376, entitled "Cross-Referenced Electrostatic Discharge Protection Systems and Methods for Power Supplies," filed Nov. 26, 1997, the entire teachings of which are incorporated herein by this reference.
Due to improvements in semiconductor technology, manufacturers can now make transistors smaller thereby reducing semiconductor size and power consumption. The decrease in transistor size has been accompanied by a decrease in transistor voltage tolerance, which is the voltage that can be applied safely across any two terminals of each transistor of the semiconductor device without causing thin oxide damage in the context of MOS-type devices, for example. This maximum tolerable voltage for the transistors is commonly referred to as the rated or process technology voltage. For example, older semiconductor devices were built using a 5V process technology where each transistor could tolerate an operating voltage of 5 Volts (V) across any two terminals without sustaining thin oxide damage. More recently, semiconductor devices have been built using a 3.3V process technology. In such devices, the voltage across any two terminals of each transistor must be less than 3.3V in order to avoid causing thin oxide damage. Presently, manufacturers are implementing 2.5V and 1.5V process technologies, and such improvements in semiconductor technology are expected to continue.
Occasionally, manufacturers combine IC's having different semiconductor technologies on the same circuit board or in the same system. For example, a manufacturer may mix some IC's having semiconductor devices built using a 5V process technology with other IC's having semiconductor devices built using a 3.3V process technology in order to obtain some of the benefits of using 3.3V process IC's (e.g., smaller packaging, lower power consumption, greater speed, lower cost). For this reason, an IC containing a semiconductor device using a 3.3V process technology must often be designed to interface with IC's containing semiconductor devices built using a 5V process technology. Specifically, the 3.3V IC must drive and receive signals at the logic levels expected by the 5V IC's in the system. To accomplish this, the 3.3V IC often requires a 5V power supply to power the 3.3V IC's I/O stages. Therefore, the 3.3V IC contains a mixture of 3.3V and 5V circuits.
Providing ESD protection in a mixed voltage IC tends to complicate the design of the ESD clamp and its control circuit. For example, one known semiconductor device includes a cantilevered ESD clamp and an RC-timed control circuit, which is interconnected between the power supply pad and the ESD clamp, to control deactivation of the ESD clamp. When power is off, the RC-timed circuit maintains ESD clamp in a conductive state for a time period related to the circuit's time constant. This allows the shunting of a short ESD event from the pad to a reference conductor. In contrast, when power is on, the RC-timed circuit operates as a voltage divider to divide a 5 V power supply signal down to a 3.6 V signal, which is used to disable the ESD clamp. Without the reduction in voltage from 5 V to 3.6 V, one or more components of the ESD clamp would be very susceptible to thin oxide damage. An example of such a circuit (hereinafter referred to as the "cantilevered circuit") is described in an article entitled "Protection of High Voltage Power and Programming Pins," by Maloney et al., EOS/ESD Symposium 97-246, (1997).